The invention concerns a process for the production of tertiary ethers (methyltertiobutylether: MTBE, ethyltertiobutylether: ETBE, methyltertioamylether: TAME, and ethyltertioamylether: ETAE) from a hydrocarbon cut comprising hydrocarbons containing four or five carbon atoms, some of them olefins.
These ethers are introduced into petrols (gasoline) to increase the octane number. The current world demand for MTBE, for example, is increasing at a rate of 25% per annum. This demand cannot be met using iso-olefins containing a tertiary carbon atom which are readily available from existing olefin production units and fluidised bed catalytic crackers. All available olefins must be considered for isomerisation to tertiary olefins, the reactive form, and thus satisfy the future demand for tertiary ethers.
The development of skeletal isomerisation technology now means that this objective can be reached. The combination of the synthesis of tertiary ethers by the reaction of tertiary iso-olefins with methanol or ethanol, and skeletal isomerisation of olefins which do not or only slightly etherify, thus merits all the attention of industrial developers.
The prior art is illustrated in the following patent documents: FR-A-2 527 202, FR-A-2 614 297, FR-A-2 527 201, FR-A-2 520 356, GB-A-2 068 408, and in the publication Chemical Economy and Engineering Review, vol. 14, No. 6, June 1982, pp 35-40 (Integrated isomerisation/MTBE production block flow diagram).
Skeletal isomerisation technology is known not to be without physical and thermodynamic constraints. Only 30% to 50% conversion is possible, with a selectivity of 75% to 85%, necessitating substantial recycling in order to convert all the olefins into tertiary isoolefins. In addition, in order to achieve the performance mentioned, the process must be carried out at rather low pressures, necessitating the use of large equipment to handle the effluent volumes which, in turn, must be compressed for product separation and recovery.
A further obstacle to the efficient combination of skeletal isomerisation and tertiary ether synthesis results from the fact that only the olefin fraction is converted during skeletal isomerisation. Saturated hydrocarbons pass through the isomerisation reactor as if they were inert compounds and are recovered with the isoolefin compounds produced, permanently diluting the reactive hydrocarbons.
The composition of C.sub.4 or C.sub.5 hydrocarbon cuts from olefin production units and catalytic crackers are very variable due both to their iso-olefin content and to the nature of the olefins present. Effluents from tertiary ether synthesis are thus very different and downstream refining arrangements must be altered as a consequences. Effluents from olefin production units, for example, are rich in linear olefins and poor in saturated hydrocarbons, and are therefore considered to be good isomerisation feedstocks. On the other hand, C.sub.4 and C.sub.5 effluents from catalytic cracking generally contain more than 50% of saturated hydrocarbons and require downstream equipment which is very bulky which also consumes enormous amounts of energy.
In addition, whatever the quality of the effluents, the skeletal isomerisation product cannot be recycled upstream of the tertiary ether synthesis reactor without a substantial purge of the accumulated saturated hydrocarbons, particularly in the case of feedstocks from catalytic cracking. This purge step thus becomes a major obstacle to the synthesis process. One of the objects of the invention is to overcome the problems or limitations described above, in particular to eliminate at least a portion of the saturated hydrocarbons present in the feedstock before the isomerisation step.